There is an old "coffee table" book called "Guns" by Dudley Pope. There is not much on small arms (more on artillery) but good illustrations and diiagrams about how guns work and their history.

Above that level, gun books tend to become specialized, so much so that few people seem interested in general knowledge. They will fix on some part of the hobby (like AR-15 rifles or single action revolvers) and just ignore everything else.